Fine morphology of the otoconial membrane and its relationship to the sensory epithelium

The ultrastructure of the otoconial membrane and its relationship to the sensory cilia of receptor cells in guinea pigs, chinchillas, rats, kangaroo rats, and mice are investigated using LM, TEM and SEM. Various tissue preparation methods are used to complement the data obtained with conventionally fixed materials. Some specimens were fresh frozen, then freeze fractured and freeze dried. Others were fixed, epoxy embedded (without curing) and freeze fractured, or fixed and critical-point dried. The otoconial membranes of the mammalian species examined demonstrated that it is made up of fibrous and amorphous (ground) substances. The following sublayers (or zones) can be identified: 1) otoconial layer, 2) gelatinous layer, 3) honeycomb layer, and 4) meshwork. The gelatinous layer seals the entire margin of the sensory epithelium; however, there are perforating holes in the striola. The honeycomb layer can be considered an integral part of the gelatinous layer, even though the gelatinous layer appears more fibrotic than the honeycomb layer. The dome-like meshwork is the structure that houses the sensory ciliary bundles and anchors the base of the honeycomb to the supporting cells. The sensory cilia, especially the tall ones of type I cells, are occasionally inserted into a chamber of honeycomb, particularly in the peripheral and parastriolar region where the honeycomb layer is thick. It is also observed that some cilia are freestanding in the striola, where the honeycomb layer is poorly developed and numerous perforating holes are present.